1. Field of the Invention
The present invention relates to a slot type curtain coating method and a slot type curtain coating device.
2. Description of the Related Art
In a slot type curtain method using a slot type die, a direction of discharging coating liquid from the slot type die is a direction of gravitational force (downward), and there is no slide part as in a slide type die, and there is no formation of a boundary layer by the slide part. Therefore, there hardly arises a problem of non-uniformity in film thickness in its width direction due to an increase in difference in film thickness between the thick film part and the thin film part in the vicinity of the both ends of the slide part. Therefore, by combining edge guides having appropriate structures and performances therewith, it is possible to form a curtain film whose fall velocity is uniform in its width direction, making it possible to obtain a coating film whose film thickness is uniform in the width direction. Further, because a shape of the die is relatively simple, which is easy to be processed, there is the feature that it is inexpensive.
However, in the slot type curtain method using the slot type die, because the direction of discharging coating liquid from the slot type die is the direction of gravitational force (downward), it is necessary to supply a large volume of coating liquid to the die at the time of replacing the air in a manifold of the die by the coating liquid at the start of coating. In a case of a single layer slot type curtain method, the coating liquid may be recovered to be reutilized. Meanwhile, in order to avoid the risk of contamination, the coating liquid may not be reutilized, but discarded in some cases. Further, air bubbles may be mixed into the coating liquid in the course of recovering the coating liquid after a coating liquid is discharged from the slot type die in some cases. However, because it is difficult to remove small air bubbles by existing defoaming devices and methods, there is the problem that it is necessary to discharge and recover a large volume of coating liquid in a work for replacing the air in the manifold of the slot type die by the coating liquid.
In order to solve the aforementioned problem, a method of providing an air exhaust vent is proposed as a method for replacing the air in the manifold of the slot type die in a slot type curtain method by the coating liquid (see Japanese Patent Application Laid-Open (JP-A) No. 05-15828). In this proposal, an air-bleeding outlet opening is provided to the manifold of the slot type die, and an antifoaming net is provided downstream thereof, and large-size air bubbles which do not pass through the antifoaming net are exhausted from the air-bleeding outlet opening. However, in this case, in order to replace the air in the manifold of the slot type die by the coating liquid, the coating liquid is discharged from the discharge opening of the slot type die to raise the liquid level in the manifold by its pressure loss, and the air is exhausted from the air-bleeding outlet opening by the pressure. Therefore, it is necessary to discharge a significant volume of coating liquid from the die, to recover or discard it. In particular, there is the problem that it is necessary to defoam air bubbles in a case of recovering high-viscosity coating liquid.
Further, because the direction of discharging coating liquid from the slot type die is the direction of gravitational force (downward) during suspension of coating, the coating liquid is leaked out little by little from the discharge opening of the die. As a result, air bubbles intrude or are accumulated in the manifold of the slot type die, and the air bubbles are broken in a lip part of the slot type die at the restart of coating, to contaminate the die lip to create coating streaks.
Further, when a liquid feed rate at the start of coating is low, the coating liquid falls while oscillating from side to side in a comb shape from a discharge opening of the slot type die. At this time, because the coating liquid in the vicinity of the both ends of the discharge opening of the slot type die oscillates from side to side, edge guides provided to the both ends in the width direction of the discharge opening of the slot type die or the flowing-down surface of supplemental water from the edge guides may be contaminated. In this state, flowing-down of the supplemental water is disturbed, so that the fall velocity at the edges of the curtain film decreases to deteriorate the uniformity in film thickness. Also, when a curtain film is formed with the edge guides contaminated, the formed curtain film warps or does not fall vertically, thereby generating streaks so as to draw a circular arc ranging from contact portions between the curtain film and the edge guides to the impact area between the curtain film and a support medium. Corrugated turbulence is caused at the impact area, which problematically deteriorates the uniformity in film thickness.
Moreover, because the direction of discharging coating liquid from the slot type die is the direction of gravitational force (downward), it is hard to check the position of the slot in the case where a cleaning reed-shaped film is inserted into the slot of the slot type die to clean it. Therefore, it is difficult to clean the slot of the slot type die. Further, when a cleaning reed-shaped film is inserted upward into the slot to clean it, the arms of a worker are upward, and the cleaning liquid or cleaning waste liquid flows down along the arms of the worker. Therefore, there is the problem that at least one of the arms and the clothes of the worker is contaminated with the cleaning liquid or the cleaning waste liquid.
Further, as a method for facilitating cleaning of the slot and the lip of the slot type die of the slot type curtain coating device, for example, it has been proposed to support the right and left both ends of the die rotatably with respect to a support frame (see JP-A No. 10-5658). However, this proposal focuses on merely facilitation of cleaning of the slot and the lip of the slot type die, and does not intend to improve uniformity in coating, and to solve various problems caused at the start of coating of the slot type curtain coating method and during temporary suspension of coating.